This invention relates to air sampling and filtration systems and more particularly to methods and apparatus for removing compounds from air to clean the air and for providing quantitative analyses of the content of particular organic compounds in the air.
Devices are known for filtering air and for sampling air to identify types and levels of organic compounds present in the environment from a variety of sources such as the manufacture and use of chemicals, combustion of fuels, and others. Adsorbent materials are often employed in such devices to filter polluted air or trap measurable quantities of pollutants for analysis. However, while qualitative analyses of air are possible with these instruments and techniques, accurate quantitative analyses have proven difficult to achieve, particularly when the air to be analyzed contains complex organic compounds in low concentrations. One problem with use of adsorbent materials is that some adsorbents such as activated charcoal vary substantially in composition among manufacturers and between batches and also do not trap effectively certain low molecular weight compounds of interest. Moreover, after compounds are trapped, heat is commonly used to desorb the compounds from the adsorbent for analysis, and this frequently results in the formation of additional amounts of the compounds whose detection is sought or alteration of those trapped. Use of sophisticated and expensive materials such as gas chromatographic materials as adsorbents helps avoid variations in composition but not the heat desorption problem and may also involve "breakthrough" problems--i.e. during trapping certain compounds to be separated from the air gradually migrate through the gas chromatographic material in the direction of airflow and are lost from the collection device. Loss of a portion of the compounds introduces errors in a quantitative analysis and could also be a drawback to use of these materials in filtering apparatus such as gas masks or chemical hoods. Certain other adsorbents such as silica gel may have at least one of the above-mentioned drawbacks and in addition exhibit an affinity for moisture which degrades their trapping efficiency and renders them unsuitable for analysis or filtering of moist air.
Among the substances whose detection in air and/or removal therefrom are of importance are pesticide residues, compounds in vapors from explosives such as TNT, nitroglycerine, C4, and RDX, industrial pollutants such as nitrobenzene, vinyl chloride, acetonitrile, and acrylonitrile, organic compounds present in automobile and truck emissions and in cigarette smoke, and others. Of particular interest in the present invention, in addition to these compounds, are N-nitroso compounds or N-nitrosamines, each of which have the general formula ##STR1## where R.sub.1 and R.sub.2 are the same or different organic radicals including those radicals which together with the depicted N-NO molecular bond constitute a nitrogen heterocyclic radical. These N-nitroso compounds include as a group several which are known to be potent carcinogens, and dosages of a few parts per million of these N-nitrosamines have produced cancers in animals. The high carcinogenity of these compounds when inhaled or ingested has caused much concern regarding their occurrence in certain materials and in the environment.
In the continuing search for the presence of these N-nitroso compounds and their possible role in causing cancer in humans, considerable interest has developed in monitoring air such as that in or near factories and cities where volatile N-nitroso compounds have been found and non-volatile N-nitroso compounds may also occur under certain conditions. However, in contrast to foods and chemicals, samples of which have readily been obtained for measurement of the content of N-nitroso compounds, accurate collection and analysis of samples from atmospheric environments is considerably more difficult as mentioned above in the general discussion of air sampling and filtering. This is true particularly if rapid collection and analysis are desired to monitor air whose quality is susceptible to change and/or to monitor air by means of devices sufficiently portable to be worn by personnel. It is important, for example, that the technique and apparatus used to filter or test air for the presence of N-nitrosamines or other organic compounds, whether or not part of a portable system, permit trapping and retention of substantially all of the predetermined compounds in the air sampled, and quite desirable that the system of which they are part provide accurate and rapid analysis of the compounds extracted from the air.
One prior method of detecting N-nitroso compounds in air is to bubble air through a solution of KOH (potassium hydroxide), extract the contents of the trap using a solvent, dry and then concentrate the extract, and then analyze the extracted sample for the presence of N-nitroso compounds. One major disadvantage of this method is that collection efficiency is low for certain N-nitroso compounds of interest such as N-nitroso-dipropylamine, over 80 percent of which may pass through the bubble trap. Also, this method requires drying and concentration of the extract as by evaporation, which necessitates precise measurements and controls, is time consuming, and may introduce errors in the analysis. The collection apparatus utilized in this technique is also too cumbersome for use in a personnel monitor and requires caustic solutions whose preparation, use, and disposal present hazards to personnel and to equipment such as air pumps.
Another method which has been used to monitor air for N-nitroso compounds is to draw air through cartridges containing Tenax GC material, a gas chromatographic material available from Applied Science Laboratories, Inc., State College, Pa., desorb the trapped materials using heat, then analyze the desorbed materials as by capillary gas-liquid chromatography followed by low-resolution mass spectrometry. However, not only is Tenax GC material quite expensive but if desorption by heat is used after collection inaccuracies can result since the thermal desorption may produce N-nitroso compounds from precursors such as amines and compounds containing nitrogen and/or oxygen present on the cartridge or may cause decomposition of some of the N-nitroso compounds trapped. Another deficiency of Tenax GC material is that it may not retain all of the N-nitroso compounds in the air pumped through the cartridge. The collection efficiency has been found in tests to be different for different N-nitroso compounds and after a certain volume of air has been drawn through the cartridge, further monitoring results in the "breakthrough" and loss of certain N-nitroso compounds from the cartridge. The breakthrough volume of air is different for different N-nitroso compounds, being generally lowest for the smaller N-nitroso compounds (such as N-nitrosodimethylamine) which are of maximum interest.
Accordingly, it is a general object of this invention to provide improved methods and apparatus for removing organic compounds from air.
It is a more particular object of the invention to provide a method and apparatus for determining the content of predetermined organic compounds in a sample of air.
It is another object of the invention to provide improved apparatus for filtering air to clean the air of particular compounds.
It is an object of the invention to provide a method for determining the content of predetermined organic compounds in a sample of air wherein the compounds may be trapped and then collected in a volume of solution sufficiently small so as to not require concentration of the solution during the determination.
It is also an object of the invention to provide apparatus for collecting organic compounds from air which is compact, inexpensive, portable, and simple to use.
It is a particular object of the invention to provide as improved method and apparatus for determining the content of N-nitroso compounds in air.
It is another object of the invention to provide apparatus for collecting N-nitroso compounds from air which will separate substantially all of the N-nitroso compounds from air directed through the apparatus and hold them without loss, then release substantially all of the N-nitroso compounds into a liquid solvent to form a solution analyzable for the total N-nitroso compound content without concentration of the solution.